1. Field of the Invention
The present invention relates to a cooling method of a steel plate, more particularly relates to a cooling method of a hot rolled steel plate.
2. Description of the Related Art
The process of continuously cooling hot rolled steel plate by a cooling apparatus and controlling the structure of the steel plate to produce thick-gauge steel plate having high strength and high toughness is widely used. This production process contributes to reduction of the production costs by the reduction of alloying elements and to the improvement of the welding work efficiency.
However, in this process of production, the temperatures of the front end part and the rear end part of the steel plate become lower in comparison with the center part of the steel plate in the length direction before the steel plate is transferred to the cooling apparatus. In addition, in water spray cooling in a cooling apparatus as well, there are large influences of heat transfer and heat conduction from the end surfaces, therefore an overcooling phenomenon occurs and the flatness and material properties easily become unstable.
For this reason, for example, as disclosed in Japanese Patent Publication (A) No. 60-43435, there is a proposed a method for tracking a steel plate position and masking the front end part and the rear end part of the steel plate by stopping the spray of the cooling water so as to prevent overcooling at the front end part and the rear end part.
The overcooling prevention effect on the front end part and the rear end part of steel plate by the masking method shown in the above Japanese Patent Publication (A) No. 60-43435 is great. However, this is ON/OFF control, that is, the masked parts are not sprayed with water, while the part which is not masked (also referred to as the “non-masked part”) is supplied with cooling water of the standard water density, therefore the amount of water sharply changes at the boundary parts. In particular, a large temperature difference occurs at the front end part of the steel plate.
For this reason, a difference in material quality occurs in the steel plate conveyance direction (also referred to as the “steel plate longitudinal direction”) inviting a drop in yield. The shape of the front end region is also easily degraded in comparison with the center part in the steel plate conveyance direction due to the influence of the temperature difference.